How Cool Can Cool Be?

New Research Initiatives Show the Benefits of Cool Roofs in Three Areas

By Greg Zimmerman, Executive Editor
Building Operating Management Magazine

April 2011

On July 19, 2010, Secretary of Energy Steven Chu announced that cool roofs would be mandatory on all Department of Energy (DOE) facilities. He also urged other federal agencies to follow suit. Chu, who is also a Nobel-prize winning scientist, cited a possible 10 to 15 percent reduction in building energy use, depending on the configuration of the building and other variables, "simply by having a white roof."

Reflective, white roofs can mean a 10 to 15 percent reduction

in a building's energy use, depending on the configuration

of the building and other variables.

But energy savings isn't the end of the potential benefit of cool roofs. Two further reasons Chu mentioned for his advocacy of cool roofs are the role they can play in helping to mitigate global warming and their ability to reduce urban heat islands­ — whereby dark roofs and asphalt pavements in concentrated urban areas increase the air's ambient temperature between 6 and 8 degrees compared to suburban or rural areas. From 50 to 65 percent of urban surfaces are covered with roofs or pavement, and Chu cited a Lawrence Berkeley National Laboratory (LBNL) study that says increasing the reflectance of those surfaces would have the same effect in terms of global warming mitigation as removing every car in the world from the road for 11 years.

That's an impressive number, to be sure. But further quantifying the potential benefits of cool roofing will be the goal of what will be a several-years long DOE initiative. The Cool Roofs Roadmap lays out research into the exact impact of cool roofing on three levels: trimming building energy use, mitigating the urban heat island effect and reducing global warming. The Cool Roofs Roadmap, currently in draft Version 2.0, is expected to be finalized by the end of this fiscal year, according to Marc LaFrance, building envelope program manager at DOE. The roadmap builds on current and past research to provide proof, and therefore act as a basis for policy, of cool roof benefits.

For now, however, in order to understand what's to come, let's take a look at the current state of cool roofing research and other initiatives that facility managers should know about if considering a cool roof for their own facilities.

Building Level: Energy Savings
A 2010 LBNL study showed that the average annual savings when substituting a .55 weathered reflectance cool roof for a .20 weathered reflectance gray roof is 5.02 kWh per square meter of roof surface. The study also showed that the savings are greater in areas of the country with higher air conditioning loads where an upper range benefit of 7.69 kWh per square meter was possible.

Cool roofs having greatest benefit in air conditioning-heavy climates makes logical sense. The idea also jibes with an important change to the most recent version of the ASHRAE 90.1 energy code. The 2010 version makes cool roofs a prescriptive requirement for Climate Zones 1 through 3. Roofs in those areas — the South, Southwest, and most of California — that adopt the code must install roofs with a minimum three-year weathered reflectance of .55, and three-year weathered emittance of .75. Infrared emittance is the measure of how much radiation a surface re-emits to the atmosphere.

Using weathered results, as opposed to new, represents a shift in the industry in how cool roof effectiveness is measured, as well. New cool roofs can exhibit reflectance numbers as high as .75 or .80. Energy Star labels roofing products with initial reflectances of .65 on low slope roofs. But the industry is beginning to understand that the true measure of how well the roof will perform depends not on its initial reflectance number, but on its weathered. "Every cool roof gets soiled and loses performance over time," says LaFrance. "So it's very important to base energy calculations on aged value, not new."

That is exactly what DOE's new Roof Savings Calculator does. Facility managers can go to www.roofcalc.com, enter several variables, such as area of the country, building type, heat and cooling type, and efficiency. They also enter characteristics of their existing roof and proposed cool roof, including weathered reflectance and emittance ratings. The resulting calculation shows the potential annual energy savings (or loss) with a cool roof. According to André Desjarlais, group leader of the Building Envelope Program at Oak Ridge National Lab, the calculator was relaunched late last year to accomplish a few goals. First, he says, it combines two calculators into one. Previously, the Environmental Protection Agency (EPA) and DOE each had its own cool roof calculator. These two calculators were programmed with slightly different assumptions and therefore gave different results.

The new calculator combines research and analysis from both agencies to be more comprehensive. This is the second accomplishment of the new calculator: It's more sophisticated than either of the previous two in terms of how it allows facility managers to enter data about their facilities. Basically, there are more variables. "The old calculators didn't specify a building type," says Desjarlais. "They assumed the roof was floating above space. Now the calculator is more sophisticated. We tried to make it more thorough, to cover more scenarios."

Part of DOE's Cool Roofs Roadmap is to expand and improve the Roof Savings Calculator and to benchmark its results against actual experimental data. Another part of the plan is to study several different types of roofing, such as vegetative roofs and roof coatings, to determine energy savings potential.

City Level: Urban Heat Islands
One of the goals of the Cool Roofs Roadmap, according to LaFrance, is to answer one of the most fundamental cool roof-related questions: Are cool roofs beneficial in cold climates? Desjarlais and LaFrance both say they think so, if not from strictly an energy standpoint, then from the fact that they can help reduce the urban heat island effect.

"If you put a cool roof on a building in a cold climate and run the calculator strictly on energy, it may not save energy because of the heating penalty," says LaFrance. "But if you take into account the urban heat island effect, the indirect benefits may make cool roofs worthwhile."

This is often a tough sell for facility managers only worried about their own bottom lines. But research has already shown the effectiveness of cool roofs at reducing urban heat islands, pollution and smog. For instance, the Heat Island Group at LBNL showed that Los Angeles spends about $100 million in extra energy costs to mitigate its urban heat island.

Reducing the temperature citywide means less air conditioning energy is required everywhere, which also means fewer greenhouse gas emissions. For this reason, many northern cities like Chicago, Philadelphia and New York City, have mandated cool roofs in their building codes. Utilities in northern states like Wyoming, Minnesota and Oregon offer rebates for the use of cool roofs. And the voluntary LEED rating system specifies cool roofs for a point explicitly for "urban heat island reduction." LaFrance says he hopes the research planned in the Cool Roofs Roadmap will more firmly establish a link between widespread use of cool roofs and reduced air conditioning energy use, pollution and greenhouse gas emissions.

One specific initiative, on which DOE is already working in conjunction with two other organizations (the White Roofs Alliance and Local Governments for Sustainability), is called the 100 Cool Cities initiative. DOE hopes to engage 100 cities around the world to commit to widespread installation of reflective surfaces by 2020. New York City, Taipei and Athens are the first three cities to sign up.
Global Level: Cooling the Planet

On an even larger scale, cool roofs and other reflective surfaces could help mitigate global warming by physically cooling the planet. The LBNL study Secretary Chu cited concluded that increasing the reflectivity of roofs and pavements in all North American cities with populations greater than 1 million would achieve a one-time offset of 57 gigatons of carbon dioxide emissions — double the worldwide carbon dioxide emissions total of 2006.

"Cool roofs are one of the quickest and lowest cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change," Secretary Chu said during last summer's announcement.

The advantage of reflective surfaces as a method of physically cooling the planet is that it's actually a double-barreled attack. By reducing the temperature in urban areas, less air conditioning energy is used, and therefore fewer carbon dioxide emissions are produced. In addition, because cool surfaces reflect the sun's energy back to space, the actual temperature of the planet is lowered.

An LBNL study found that raising the Earth's average albedo (or reflectance) by 0.003 would result in an average temperature decrease of 0.01 degrees Celsius. That may sound like a modest temperature change, but it's also a relatively modest albedo change. And, as Desjarlais says, "every little bit helps."

The Cool Roofs Roadmap lays out plans to work with India and China on research to try to determine how cool surfaces can not only cool the planet, but also reduce pollution.

"If we do the research, and if the research shows what we suspect in terms of urban heat island mitigation and global cooling, policy can be based on this research," says LaFrance.